I am keen on being able to specify the function directly, rather than having to fiddle with some stencils dimensions. Is there a specific stencil that allows to type in the function, can one do that using a shape sheet or VBA?

Wow, I did know about this. In principle, Word is just as fine for me as Visio. Thanks. But the quality does not seem to be that high. Anyway, I'll leave the question open for a little while but your answer is definitely a strong candidate.
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toksingMay 1 '12 at 13:37

2D is alright, 3D isn't that great. You might want to play around a bit with the standalone Microsoft Mathematics, but the quality should be the same. If you want something better, it's time to look at more dedicated programs, such as Mathematica (not free). Geogebra is excellent for 2D and there's a beta GeoGebra 3D. All of those require copy/paste or save/import into MS Office.
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BobMay 1 '12 at 14:06